The present invention relates to a sintered body of a carbonitride alloy with titanium as main component which has improved properties particularly when used as cutting tool inserts in intermittent metal cutting operations under particularly toughness demanding conditions. This has been done by a different distribution of hard constituents and binder phase between the surface layer and inner (bulk) zone and a different form of the hard constituents in the surface zone and bulk zone in regard to simple and complex structures, particularly different core-rim-situations.
Titanium based carbonitrides (so-called cerments) are today well established in the metal cutting industry and are primarily used as tools for finishing. They consist of hard constituents of titanium-based carbonitride embedded in a binder phase of cobalt and/or nickel. The hard constituents generally have a complex structure with a core surrounded by a rim of a different composition.
For tungsten carbide-cobalt-based hard metals, the so-called gradient sintered grades, particularly when coated with one or more CVD layers, have now gained strong foothold in metal-cutting inserts. Gradient sintering means that the sintering is performed in such a way that an about 10 .mu.m wide surface zone of the material gets another composition that its inner part, particularly with a higher binder phase content in the surface zone. Examples of patents within this area are U.S. Pat. Nos. 4,277,283, 4,610,931, 4,497,874, 4,649,048, 4,548,786 and 4,830,930. U.S. Pat. No. 4,911,989 describes a coated hard metal where the hardness increases monotonously in a 50-100 .mu.m wide surface zone.
Different forms of gradient sintering for titanium-based carbonitride alloy have existed for a number of years. For example, grades exist with a few .mu.m thick coating with strong binder phase enrichment and below that a binder phase depletion which extends 200-400 .mu.m into the material with a gradual increase up the bulk level. This gradient type gives increased wear resistance which takes place with a certain loss of the toughness behavior. As expected, a hardness maximum is obtained just below the binder phase enriched zone where the enrichment of hard constituents is the greatest.
One way of improving the toughness behavior is through a relatively moderate binder phase enrichment to a depth of about 20-50 .mu.m from the surface followed by an enrichment of hard constituents which then gives a hardness maximum. The binder phase enrichment gives a better toughness behavior but increases at the same time the risk for plastic deformation. The hard constituent enrichment increases the wear resistance (when the wear has reached this area) but increases the risk of crack propagation, i.e., deteriorates the toughness behavior at the same time as the resistance to plastic deformation increases.
An example of a variant of the above is U.S. Pat. No. 5,059,491, which discloses a hard surface layer with a hardness maximum situated between 5 and 50 .mu.m from the surface and an outer surface zone with a hardness of between 20 and 90% of the maximum hardness. This is accomplished by starting the sintering process in an non-oxidizing atmosphere up to 1100.degree. C. followed by a nitriding atmosphere which is finished by a denitriding atmosphere. The denitriding period comprises at least the cooling but can also comprise the whole or part of the sintering holding time.
Thus, normal gradient sintered hard alloys get a depletion of binder phase, i.e., an enrichment of hard constituents, just below the binder phase enrichment. This leads to increased wear resistance in this area with increased resistance to plastic deformation, but unfortunately also leads to a worsened toughness behavior.